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Old 11th Dec 2017, 7:12 pm   #1
bill knox
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Default Replacement of OC171 in an Eddystone EC10

Hi

I have a Eddystone EC10 that I am going over, but I have found that several for the OC171 transistors are either intermittent or have a partial S/C from screen to the other pins.

I have seen on this forum about replacing the AFXXX range with other transistor's but although I have looked have not seen anything regarding the OC171.

Can anyone suggest a replacement for the OC171, the main problem seems to be the gain of so-called replacements, their hfe seems to be around the 50 mark where as the gain of an OC171 is rated at hfe 150.

Could I have some views on this please.

Regards

Bill
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Old 11th Dec 2017, 7:19 pm   #2
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Default Re: Replacement of OC171 in an Eddystone EC10

I would expect that the "usual suspects"- AF12x, GT322B etc would work just fine.
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Old 11th Dec 2017, 7:41 pm   #3
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Default Re: Replacement of OC171 in an Eddystone EC10

Agreed. The OC171 is essentially the same transistor as the AF11x, and the same replacement guidelines apply.

There is no one hfe value for a transistor, it all depends on the operating conditions.
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Old 11th Dec 2017, 7:58 pm   #4
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Default Re: Replacement of OC171 in an Eddystone EC10

Some people in the past have suggested using transistors like the AF139/AF239 in such applications - thinking "That's a UHF transistor - higher Ft so it's got to be better" - truth is, the Hfe of them is generally lower than the Hfe of a boring transistor like an AF114/115/124/125 "general band-II VHF-broadcast" one.
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Old 11th Dec 2017, 8:33 pm   #5
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Default Re: Replacement of OC171 in an Eddystone EC10

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Originally Posted by bill knox View Post
Can anyone suggest a replacement for the OC171, the main problem seems to be the gain of so-called replacements, their hfe seems to be around the 50 mark where as the gain of an OC171 is rated at hfe 150.
Billinsghurst (reputable long-established UK valve & transistor firm) have NOS ex WD CV7089 (OC71 equivalents) for £2.80 each.

Don't know if they're of any interest - they might be worth a punt:

https://www.ebay.co.uk/itm/CV7089-OC...-/191990595205

(Hfe isn't the be all and end all and isn't that important in some roles - ie, an oscillator needs to oscillate up to the desired frequency).

Good luck in your endeavours Bill.
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Old 11th Dec 2017, 8:39 pm   #6
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Default Re: Replacement of OC171 in an Eddystone EC10

I'd be worried that a "NOS" OC171 equivalent from the same era and heritage as the original OC171 would have the same tin-whiskerism issue that's probably the reason for the OP to need his original OC171 replacing...
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Old 11th Dec 2017, 9:04 pm   #7
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Default Re: Replacement of OC171 in an Eddystone EC10

Agreed, best to steer clear. AF12xs can be bought for less than that.
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Old 11th Dec 2017, 9:12 pm   #8
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Default Re: Replacement of OC171 in an Eddystone EC10

The EC10 has 6 OC171 transistors but only 4 of them are in the RF and IF stages where high gain is important.

It may be possible to use a lower gain device for the Local Oscillator and the BFO.

Another factor is that they are not connected to the AGC line and so it may be possible to change the biasing and use silicon PNP devices.
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Old 12th Dec 2017, 1:15 am   #9
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Default Re: Replacement of OC171 in an Eddystone EC10

I have replaced the OC171's in my EC-10 radios with AF178's with outstanding results. These have near identical and a little superior specs to the OC171, they look the part because of their larger size than an AF127's which don't look good in these radios and have short awkward lead wires.

There are also the Tesla resin filled OC169 & OC170 if you are lucky enough to get them, but they probably sold out like hot cakes.

The other excellent transistor for the application is the 2N2084, these have very long lead wires which is helpful. The last lot of these I got came from Halbleiter.

Have a look at post 5 and post 8 in this thread:

https://www.vintage-radio.net/forum/...d.php?t=139413

Don't bother replacing the OC171 with any transistor of the similar grease filled design, Tin whiskers will kill them in the end.

Since you are an EC-10 owner needing to replace transistors, you might be interested in this thread:

https://www.vintage-radio.net/forum/...d.php?t=139849

Last edited by Argus25; 12th Dec 2017 at 1:20 am.
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Old 12th Dec 2017, 6:19 pm   #10
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Default Re: Replacement of OC171 in an Eddystone EC10

GT322Bs have long leadouts (about 25mm) and are easier to fit than AF12x types.
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Old 12th Dec 2017, 7:00 pm   #11
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Default Re: Replacement of OC171 in an Eddystone EC10

Just beware that the leadout is different from AF12x!
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Old 12th Dec 2017, 7:55 pm   #12
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Default Re: Replacement of OC171 in an Eddystone EC10

Quote:
Originally Posted by Argus25 View Post
I have replaced the OC171's in my EC-10 radios with AF178's with outstanding results. These have near identical and a little superior specs to the OC171, they look the part because of their larger size than an AF127's which don't look good in these radios and have short awkward lead wires.
Over 300 in stock here at £1.25 each, which might be of interest:

http://markhindes.easywebstore.co.uk...tor_A27YN.aspx
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Old 12th Dec 2017, 8:03 pm   #13
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Default Re: Replacement of OC171 in an Eddystone EC10

Really, subbing these transistors isn't at all difficult. I have yet to hear from someone who has subbed a plausible alternative and had problems. Even going to silicon is usually trouble free.
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Old 19th May 2018, 1:19 am   #14
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Default Re: Replacement of OC171 in an Eddystone EC10

Recently bought an EC10 in immaculate condition. I know about the whisker/dendrite problem that affects early AF transistors. I wasn't surprised to find the set was dead apart from its audio stage which was noisy (hiss that wasn't affected by volume control).
The bias resistors in the first AF amp (oc71) are usually the cause of that, so I changed the appropriate resistors. That cured the audio hiss.

Next I gave the first RF amp oc171 a good flick and got a nice ear-splitting crackle. A second flick and loud hiss resulted that was volume control dependant. Next I did the same to the oscillator oc171 and the set came to life! It didn't last for long and I had to repeat the treatment. The flick had to be done quite often, so I snipped the screen lead on the oscillator and first RF transistor. That usually clears the short, at the expense of possible instability.

Since then I've been using the set for days now with no further problems. Its quite interesting that I saw problems only in the oscillator and first RF transistor. I seem to remember reading that somewhere.

I have bought a supply of AF178s to replace the oc171s in the set. It seems less sensitive than I'd expect, when compared with my 680X on the same aerial. I'm thinking that although they work, some or all of the oc171s are leaky/low gain.
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Old 27th May 2018, 10:27 pm   #15
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Default Re: Replacement of OC171 in an Eddystone EC10

I've just replaced all the OC171s in my EC10 with AF178s. The difference is spectacular, a big increase in sensitivity. Well worth doing and not difficult to do.
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Old 28th May 2018, 2:11 pm   #16
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Default Re: Replacement of OC171 in an Eddystone EC10

Yes, I suggested this a while back on the forum.

I had replaced the OC171's in my EC10 with AF178's over 10 years ago now. I was surprised that nobody in the UK had done this before.

After reviewing the specs of germanium RF transistors in the Philips manual, the AF178 was the logical alternative for many reasons (despite the fact it was designed for TV applications), especially that it just out spec'd the OC171 for its frequency response and had just a slightly lower C-B feedback capacitance and it had a good sized housing resembling the physical size of the OC171. Also the AF178's are resin filled and immune from the Tin whisker effects.

I set up a test jig to compare AF178's with OC171's, over the frequency range that they are used in, in the EC10 to 30MHz, and the AF178's were always just a bit better every time with higher gain and equal or lower noise compared to non tin whisker affected OC171's.

In any case, I'm glad this idea from down under is catching on now.

In my view the EC-10 is one of the most wonderful radios ever made. Its ratio of simplicity to performance beats many more complicated radios and the current consumption is amazingly low compared to many comms radios loaded with power hungry digital logic.

But that is not all;

The EC-10 has a wonderful attractive physical design, a high mass tuning wheel, that feels great to tune a therefore a great feeling tuning knob, and a great looking glass dial.

You could expect to see a radio like this on the instrument panel of Thunderbird One, or at least it would have looked quite at home there and for me that is pretty hard to beat. It is a real darling of a radio and an early germanium transistor British icon of a communications receiver and it deserves all the consideration you can send its way.
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Old 28th May 2018, 2:19 pm   #17
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Default Re: Replacement of OC171 in an Eddystone EC10

Quote:
Originally Posted by Argus25 View Post
I set up a test jig to compare AF178's with OC171's, over the frequency range that they are used in, in the EC10 to 30MHz, and the AF178's were always just a bit better every time with higher gain and equal or lower noise compared to non tin whisker affected OC171's.
Just a bit better per transistor, times several stages = quite a lot better


Come to think of it, I think there was a little Eddystone buried in a control console in one of Gerry Anderson's puppet series. But which series?

David
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Old 28th May 2018, 4:33 pm   #18
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Default Re: Replacement of OC171 in an Eddystone EC10

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Originally Posted by Argus25 View Post
Yes, I suggested this a while back on the forum.

You could expect to see a radio like this on the instrument panel of Thunderbird One, or at least it would have looked quite at home there and for me that is pretty hard to beat. It is a real darling of a radio and an early germanium transistor British icon of a communications receiver and it deserves all the consideration you can send its way.
I bought my EC-10 at a BVWS show. It was in its original carton with full manual and guarantee card. It was in immaculate condition. When I saw it, I really loved the look of it. What persuaded me to buy it was that I had the chance to play with it and found that the tuning knob had the "legendary smoothness" of a full sized Eddystone like my 680X.

Apart from replacing the transistors and a bit of light dusting, I haven't had to do anything to it. I have fitted it with rechargeable cells and it seems to run forever on them!

Thanks for your suggestion of replacing OC171s with AF178s. I just searched the forum on that subject and compared all the suggestions - including blowing the shorts and snipping the screen. Yours was way the best.

Love to comments about Gerry Anderson's stuff. I'd love to find the episode that might have an EC10 in it. It might have been Captain Scarlet, which I thought was a brilliant idea. I have them all on DVD so I'll look out for an EC-10!

Any thoughts as to whether IF (and/or RF) alignment would have been affected much by my replacing the transistors? I'm inclined to think not.

Attached are a few photos of my EC-10. Also one of my TV restorations showing the starting sequence of Captain Scarlet!

By the way, I live in Kings Norton, just a mile from the Eddystone factory site. Its a housing estate now - of course.

Charles
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Old 29th May 2018, 2:41 am   #19
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Default Re: Replacement of OC171 in an Eddystone EC10

Quote:
Any thoughts as to whether IF (and/or RF) alignment would have been affected much by my replacing the transistors? I'm inclined to think not.
Attached are a few photos of my EC-10. Also one of my TV restorations showing the starting sequence of Captain Scarlet!
I think the transistors OC171 & AF178 are similar enough that you will not have to worry about the alignment changing significantly. Though on the other hand, the radio can benefit from re-alignment, if you have the right gear to do it. Otherwise if it appears in good alignment, sensible to leave it.

I'm a Thunderbirds and Captain Scarlet enthusiast. Great idea to photograph the Captain on your TV restoration, it makes a difference to have interesting and historical subject matter/images like that of the period, where it might have been seen on such a TV set. I like it !

When I had finished my Conrac Avionics video monitor project I photographed screen images from Voyage to the Bottom of the Sea & The Time Tunnel, other favorites of mine from that era, you can see the screen images on about pages 21 & 22 of this article:

http://worldphaco.com/uploads/The_19...o_monitor..pdf

Somehow they show off the vintage nature of the monitor and make to look more interesting. Of course being an avionics grade monitor the contrast, brightness, resolution and focus are mind blowingly good and the images are wonderful.
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Old 29th May 2018, 3:22 pm   #20
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Default Re: Replacement of OC171 in an Eddystone EC10

Quote:
When I had finished my Conrac Avionics video monitor project I photographed screen images from Voyage to the Bottom of the Sea & The Time Tunnel, other favorites of mine from that era, you can see the screen images on about pages 21 & 22 of this article...
Nice monitor and I love the photos. I too watched Voyage to the Bottom of the Sea and The Time Tunnel I have the full set of DVDs of both of those. We seem quite alike in that sort of thing so I suspect you might also have liked the early b/w Dr Who (or should I say Doctors Who)?

As to alignment of my EC-10, yes I do have the equipment. I have Siglent sig gen with wobulator capability and a Rigol 100Mhz scope. I'm more worried about damaging the IFT cores when adjusting, so I think I'll leave it alone. I did check the dial calibration with the sig gen and its remarkably good on all ranges.

Charles

Last edited by space_charged; 29th May 2018 at 3:31 pm.
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